This invention generally relates to ultrasound color flow Doppler imaging of fluid flow fields. In particular, the invention relates to a method and an apparatus for improving the display of such imaging by calculating, a threshold.
Ultrasonic scanners for detecting blood flow based on the Doppler effect are well known. Such systems operate by actuating an ultrasonic transducer array to transmit ultrasonic waves into the object and receiving ultrasonic echoes backscattered from the object. In the measurement of blood flow characteristics, returning ultrasonic waves are compared to a frequency reference to determine the frequency shift imparted to the returning waves by flowing scatterers such as blood cells. This frequency, i.e., phase shift, translates into the velocity of the blood flow. The blood velocity is calculated by measuring the phase shift from firing to firing at a specific range gate.
The change or shift in backscattered frequency increases when blood flows toward the transducer and decreases when blood flows away from the transducer. Color flow images are produced by superimposing a color image of the velocity of moving material, such as blood, over a black and white anatomical B-mode image. Typically, color flow mode displays hundreds of adjacent sample volumes simultaneously, all laid over a B-mode image and color-coded to represent each sample volume""s velocity.
Typically, color flow processors estimate blood flow velocity, blood flow power Doppler imaging (PDI) and blood flow variance. Typically, color flow data is used to modify the color of a region of interest on a display screen. The user selects the type of data used for the display. The modes typically available are power only, velocity only or velocity and variance combined.
When operating in the color velocity or power Doppler imaging (PDI) modes on an ultrasound scanner, sometimes the color spills out of the vessel and writes on the vessel walls or surrounding plaque and tissue. This is an artifact caused by the fact that the color firings and the b-mode firings are independent of each other and that the resultant color flow image must be overlayed into the b-mode image. Differences in spatial and temporal resolution and registration between color and b-mode contribute to this articraft. The effectiveness of color flow wall filtering and power thresholding can also contribute to this artifact. The current LOGIQ 700 product manufactured by General Electric has a B/Color priority threshold which helps to minimize this artifact. For a given display pixel location, when the b-mode gray scale level is above this B/Color priority threshold, the b-mode or gray scale pixel will be written to that pixel location on the display. Otherwise color will be written if there is a valid color value associated with that pixel. Plaque, tissue, and vessel walls typically have a higher gray scale level than those regions containing real flow. The B/Color priority threshold, therefore, helps to eliminate color bleeding onto these higher gray scale regions. The problem is that B/Color priority threshold is typically preset to some level which may or may not be adequate for any particular patient anatomy or user settings such as gain and dynamic range. Even if this threshold can be manually adjusted by the user, the typical user is not likely to be willing to search for the best threshold, and the resolution of the threshold selections may not be adequate to start with.
Accordingly, there is a need for a color flow ultrasound scanner which can automatically adjust the B/color priority threshold according to the actual B-mode data.
In the color flow power or color flow velocity modes of operation, known ultrasound scanners provide user selectable color maps that are applied to the color mode data. In the power mode, the possible data values are 7-bits (0-127). In the velocity mode, the data values are signed 7-bits (xe2x88x92128 to +127). In a known system, any given map is fixed with the colors in the map being applied across the range of data values for power or velocity modes. There are also fixed map thresholds which are set such that color data will be displayed if it is above this threshold and will not be displayed if it is below this threshold. Such thresholding typically is used to minimize unwanted noise and artifacts. The fixed map threshold may be too high for some scanning situations, causing low flow to be thresholded out even if no artifacts are present. As a result, there is a need for a color flow system in which the map and map threshold can be adjusted automatically.
The preferred embodiment is useful in an ultrasound imaging system generating color flow signals and b-mode signals in response to ultrasound signals backscattered from a subject under study. In such an environment, improved image display can be achieved in response to the color flow signals and b-mode signals by receiving and altering a conventional threshold signal. Color flow data is stored in response to the color flow signals and corresponding b-mode data is stored in response to the b-mode signals, preferably in a memory. An identification of valid color flow data is obtained and an analysis of b-mode data corresponding to the valid color flow data is conducted, preferably by a logic unit. The value of the threshold signal is altered in response to the analysis. A color flow image is displayed in response to the altered threshold signal, preferably on an ultrasound display.
By using the foregoing techniques, a color flow display can be superimposed on a b-mode display while minimizing color bleeding artifacts.